The present invention generally relates to thermal asperity detection methods, thermal asperity elimination methods, magnetic disk units and retry methods therefor. More particularly the present invention relates to a method of detecting thermal asperity in a magnetic disk unit, a method of eliminating the thermal asperity, a magnetic disk unit having means for detecting and eliminating the thermal asperity, and a retry method which carries out a retry by detecting the thermal asperity in the magnetic disk unit.
Recently, a magnetic disk unit having a magneto resistance effect type (MR) head which uses a magneto resistance (MR) element has been proposed. In addition, as the recording density of a magnetic disk increased, the floating distance by which the MR head floats from the magnetic disk has become smaller. For this reason, the MR head may hit a projection, that is, a defect, inevitably existing on the magnetic disk. When the MR head hits such a projection on the magnetic disk, heat is generated by friction, and a signal waveform reproduced from the magnetic disk by the MR head fluctuates due to the generated heat. Such a phenomenon is referred to as "thermal asperity".
The memory capacity of the magnetic disk unit has increased in recent years, and the increased memory capacity is mainly due to the increased recording density of the magnetic disk. In order to increase the recording density of the magnetic disk, there is a method of increasing the number of data tracks in a radial direction of the magnetic disk, and there is a method of increasing the recording density in a circumferential direction of the magnetic disk.
The MR head is suited for use with the magnetic disk using the latter method to increase the recording density. In addition, in order to further increase the recording density, there are proposals to reduce the floating distance of the MR head from the surface of the magnetic disk so that the signal-to-noise (S/N) ratio of an output of the MR head is increased.
The MR head has a characteristic such that the electrical resistance of the MR head changes depending on changes in an external magnetic field. Utilizing this characteristics of the MR head, the MR head reads magnetizations on the magnetic disk as voltage signals by applying a constant current to the MR element. Further, unlike an inductive type head, the MR head can easily read signals even while the magnetic disk rotates at a low speed, and the MR head is thus suited for use in the magnetic disk unit to increase the memory capacity and to reduce the size of the magnetic disk unit.
However, when the floating distance of the MR head from the surface of the magnetic disk is reduced in order to increase the recording density on the magnetic disk, the MR head hits projections, that is, defects, inevitably existing on the magnetic disk. When the MR head hits such a projection, the thermal resistance of the MR element changes, that is, increases, due to the heat generated by the friction. When the thermal resistance of the MR element increases, the signal waveform reproduced from the magnetic disk by the MR head fluctuates, thereby causing the thermal asperity. More particularly, when the thermal asperity occurs, a sudden change occurs in a D.C. component of the signal waveform reproduced from the magnetic disk, and it becomes impossible to correctly reproduce data recorded on the magnetic disk.
Conventionally, there was a proposed method which takes measures to suppress the undesirable effects of the thermal asperity by detecting the thermal asperity and increasing an input dynamic range of an analog-to-digital (A/D) converter within a data read part. According to this proposed method, the operations of a phase locked loop (PLL) and an automatic gain control (AGC) loop within the data read part are held, and a cutoff frequency of an A.C. coupling of the input of the data read part is increased. This proposed method is disclosed in a Japanese Laid-Open Patent Application No.6-28785, for example.
However, according to the conventional measures taken against the thermal asperity, there was a problem in that it is impossible to completely eliminate the sudden change in the D.C. component of the signal waveform reproduced from the magnetic disk. In addition, when carrying out a retry of the read operation after detecting the thermal asperity, it was necessary to carry out 3 operations, namely, holding the operations of the PLL and the AGC loop, increasing the input dynamic range of the A/D converter and increasing the cutoff frequency of the A.C. coupling of the input of the data read part. As a result, there was also a problem in that a complicated control is required.
Furthermore, when the operations of the PLL and the AGC are held, it is impossible to follow the data during the hold time. But since a deviation of the signal level from a target value and an error in the sampling phase will cause data error, it was necessary to minimize the hold time. On the other hand, when the input dynamic range of the A/D converter is increased, the resolution deteriorated. Moreover, an analog circuit that is used to carry out a switching when increasing the cutoff frequency of the A.C. coupling at the input of the data read part is in general easily affected by noise, and it was essential to take measures against the noise.